The present invention concerns a coated cemented carbide cutting tool insert particularly useful for turning of cast irons. The insert has a body with a tough Co binder phase, WC and cubic carbonitrides as hard phases, and a wear resistant coating. The surface zone of the insert body is of a different elemental composition than the bulk composition, yielding simultaneously good wear resistance, plastic deformation resistance and edge toughness.
Coated cemented carbide inserts with binder phase enriched surface zones are used for machining of steel and stainless steel materials. The binder phase enriched surface zone widens the application area towards tougher cutting operations. In inserts for turning of cast irons these cemented carbide grades have usually not shown good performance. Cemented carbide grades dedicated for machining of cast irons have traditionally been designed with low Co content, small WC grain size, and no or very small additions of cubic carbides, for the reason of WC grain growth inhibition only. Such cutting tool materials have relatively high room temperature hardness, fair crack propagation resistance and bulk toughness properties. However, in difficult applications demanding a high amount of toughness due to non-continuous cuts or more difficult to machine cast irons, the traditional grades are too brittle, resulting in edge chipping or insert breakage, one consequence being a lower productivity due to the need to use more moderate cutting data.
The cemented carbides in the traditional grades for machining cast irons sometimes have limited plastic deformation resistance, and in some operations also limited wear resistance. To improve these properties, a decrease of the WC grain size and lowering of the Co binder phase content and/or an increased addition of cubic carbonitride forming elements is needed. Each of these changes decreases the insert toughness.
Methods to improve the toughness behavior by introducing an essentially cubic carbide free and binder phase enriched surface zone are known. U.S. Pat. Nos. 4,277,283, 4,610,931 and 4,548,786 describe methods to accomplish binder phase enrichment in the surface region by dissolution of cubic carbide phase close to the insert surfaces. The methods require that the cubic carbide phase contains some nitrogen, since dissolution of cubic carbide phase at the sintering temperature requires a partial pressure of nitrogen, nitrogen activity within the body being sintered exceeding the partial pressure of nitrogen within the sintering atmosphere. The nitrogen can be added through the furnace atmosphere during the sintering cycle and/or directly through the powder. The dissolution of cubic carbide phase, preferentially in the surface region, results in small volumes that will be filled with binder phase giving the desired binder phase enrichment. As a result, a surface zone consisting of essentially WC and binder phase is obtained.
U.S. Pat. No. 6,333,100 relates to a coated cemented carbide insert for turning of steels. The insert has a highly alloyed Co-binder phase, from about 4 to about 12, preferably from about 7 to about 10, percent by weight of cubic carbides and a WC grain size of from about 1 to about 4, preferably from about 2 to about 3 μm. The binder phase enriched surface zone is of a thickness <20 μm and along a line in the direction from the edge to the center of the insert the binder phase content increases essentially monotonously until it reaches the bulk composition. The coating of the insert comprises from about 3 to about 12 μm of columnar TiCN and from about 2 to about 12 μm of Al2O3.
U.S. Pat. No. 5,945,207 describes a cutting tool insert particularly useful for cutting of cast iron materials. The insert is characterised by a WC—Co cemented carbide body with from about 5 to about 10 wt. % Co and <0.5% cubic carbides from groups IVb, Vb or VIb of the periodic table. The binder phase is highly W-alloyed and the surface composition is well defined. The coating comprises a layer of TiCxNyOz with columnar grains, a layer of fine-grained textured α-Al2O3 and a top layer of TiCxNyOz that has been removed along the edge line.